diff --git a/lib/ast.ml b/lib/ast/print.ml
similarity index 51%
rename from lib/ast.ml
rename to lib/ast/print.ml
index c7a5d47..17d3048 100644
--- a/lib/ast.ml
+++ b/lib/ast/print.ml
@@ -1,33 +1,35 @@
-(* lib/ast.ml *)
+(* lib/ast/print.ml *)
 
-type binary_operator = Add | Substract | Multiply | Divide
-
-type expression =
-  | IntegerLiteral of int
-  | BinaryExpression of binary_operator * expression * expression
-
-type statement = ExpressionStatement of expression
-type source_file = SourceFile of statement list
-
-(* Print AST *)
+open Syntax
 
+(** [string_of_binary_operator op] returns a string representation of the binary operator [op]. *)
 let string_of_binary_operator = function
   | Add -> "Add"
   | Substract -> "Substract"
   | Multiply -> "Multiply"
   | Divide -> "Divide"
 
+(** [string_of_unary_operator op] returns a string representation of the unary operator [op]. *)
+let string_of_unary_operator = function Negate -> "Negate"
+
+(** [string_of_expression e] returns a string representation of the expression [e]. *)
 let rec string_of_expression = function
   | IntegerLiteral i -> "IntegerLiteral(" ^ string_of_int i ^ ")"
+  | UnaryExpression (op, e) ->
+      "UnaryExpression("
+      ^ string_of_unary_operator op
+      ^ ", " ^ string_of_expression e ^ ")"
   | BinaryExpression (op, e1, e2) ->
       "BinaryExpression("
       ^ string_of_binary_operator op
       ^ ", " ^ string_of_expression e1 ^ ", " ^ string_of_expression e2 ^ ")"
 
+(** [string_of_statement s] returns a string representation of the statement [s]. *)
 let string_of_statement = function
   | ExpressionStatement e ->
       "ExpressionStatement(" ^ string_of_expression e ^ ")"
 
+(** [string_of_source_file f] returns a string representation of the source file [f]. *)
 let string_of_source_file = function
   | SourceFile stmts ->
       let stmt_strings = List.map string_of_statement stmts in
diff --git a/lib/ast/syntax.ml b/lib/ast/syntax.ml
new file mode 100644
index 0000000..126bb37
--- /dev/null
+++ b/lib/ast/syntax.ml
@@ -0,0 +1,12 @@
+(* lib/ast/syntax.ml *)
+
+type binary_operator = Add | Substract | Multiply | Divide
+type unary_operator = Negate
+
+type expression =
+  | IntegerLiteral of int
+  | UnaryExpression of unary_operator * expression
+  | BinaryExpression of binary_operator * expression * expression
+
+type statement = ExpressionStatement of expression
+type source_file = SourceFile of statement list